This invention relates to phase locked loop frequency synthesizers which can be made to hop very rapidly and accurately through a sequential and predetermined frequency schedule.
Fast frequency hopping synthesizers have their primary application in frequency agile radio frequency transmission systems where they are used for the generation of frequency signals in accordance with a predetermined frequency schedule. Frequency synthesizers employing indirect synthesis techniques have been used in such applications but are limited in the speed in which transitions between frequencies can be made to occur due to fundamental bandwidth constraints of phase locked loops used in such frequency synthesizers. Consequently, present fast frequency hopping synthesizers employ direct synthesis techniques which comprise a combination of switching and arithmetic manipulations such as mixing, multiplication and division to provide the desired rapid frequency hopping. The resulting frequency signals generated by these latter named frequency hopping synthesizers have relatively poor spectral purity due to the difficulty of eliminating undesired spurious mixer products, and other problems associated with such synthesizers such as switch leakage. In addition, fast frequency hopping synthesizers using such a direct synthesis implementation tend to be large and expensive due to complex filtering, shielding and radio frequency interference elimination requirements. Fast frequency hopping synthesizers using phase locked loop techniques solve many of the aforementioned problems associated with synthesizers operating in accordance with direct synthesis techniques. However, standard phase locked loop synthesizers are generally handicapped in their use in fast frequency hopping synthesizers, as previously mentioned, because of the limited speed at which they permit transitions between frequencies to be made.